An indolyl derivative has been used as a color developing substrate in immunoblotting or immunochromatography (see Methods in Enzymology, Vol. 121, pp. 497-509 (1986) and JP-A-9-133681 which corresponds to EP-A-0762123, the term "JP-A" as used herein means an "unexamined published Japanese patent application"). In these techniques, the activity of an enzyme immobilized on a nitrocellulose membrane through immune reaction is determined by the use of an indolyl derivative. The indolyl derivative reacts with the immobilized enzyme to form an indigoid dye, which is deposited on the membrane with the passage of time to visualize the reaction product.
Immunoblotting requires 20 to 60 minutes from the start of the reaction between an indolyl derivative and an enzyme to color detection, and it has been desired to shorten the reaction time. In immunochromatography, it takes at least 10 minutes, usually from 15 minutes to several hours, for dot blotting a membrane with a sample, development with a developing solution containing an indolyl derivative, and measuring the color developed at the detecting portion to obtain a result. In order to take full advantage of the simplicity characteristic of immunochromatography in urgent examination, it has also been desired to further shorten the time of assaying.